Dbl oncogene genesis involved the loss of the first 497 amino acids of proto-dbl and the acquisition of a new N-terminus from an unknown human gene. Both the dbl oncogene and proto-dbl products can transform NIH/3T3 cells when expressed under the influence of retroviral long terminal repeat (LTR). However, the transforming activity of dbl was significantly higher. To assess which of the structural alterations in dbl is responsible for this enhancement, two deletion mutants were constructed, one from proto-dbl and one from dbl in which only the sequences which are shared by both were kept. The mutants were cloned in an LTR-based expression vector and their transforming ability was determined in the transfection assay. Both mutants transformed NIH/3T3 cells at similar efficiency, which was as high as that of the dbl oncogene product. The subcellular localization of the mutant protein was similar to that of the parental gene. In contrast to the parental gene products, none of the mutant protein was phosphorylated. In order to search for evidence of proto-dbl involvement in human malignancies, the expression of proto-dbl-related transcript was surveyed in human tumors. Five of 11 leiomyosarcomas were positive, while tissues derived from normal smooth muscle were negative.